Then go from there. I use a cassette adapter in my (rather old) car. I use an FM transmitter on my wife's car (no cassette player, just FM and CD). Of the two, the cassette adapter is much easier - especially on road trips when the usable FM signal gaps change.

Line in is almost always the best option. About the only thing to worry about is a ground loop adding noise (usually a hum or whine) to the mix.

I have three options in my car. I can use a hands free speaker device ( I have a Jabara) and bluetooth to it and it transmits an FM signal to the car stereo. It's OK, but the sound deteriorates somewhat. second: I can use a patch cord and plug my phone into the audio system. third: I load my songs onto a thumb drive and use the USB plug into my audio system. The last two have equal tone quality outputs. A lot of cars have have the options of bluetooth, patchcord, or USB. My car is a 2011 and isn't even the "top of the line".